  <div>
    <div>

    <div class="entry">
      <form>
        <select id="mark" name="mark">
          <option value="">--</option>
          <option value="bmw">BMW</option>
          <option value="audi">Audi</option>
        </select>
        <select id="series" name="series">
          <option value="">--</option>

          <option value="series-1" class="bmw">1 series</option>
          <option value="series-3" class="bmw">3 series</option>
          <option value="series-5" class="bmw">5 series</option>
          <option value="series-6" class="bmw">6 series</option>
          <option value="series-7" class="bmw">7 series</option>

          <option value="a1" class="audi">A1</option>
          <option value="a3" class="audi">A3</option>
          <option value="s3" class="audi">S3</option>
          <option value="a4" class="audi">A4</option>
          <option value="s4" class="audi">S4</option>
          <option value="a5" class="audi">A5</option>
          <option value="s5" class="audi">S5</option>
        </select>
        <select id="model" name="model">
          <option value="">--</option>

          <option value="3-doors"       class="series-1">3 doors</option>
          <option value="5-doors"       class="series-1">5 doors</option>
          <option value="coupe"         class="series-1">Coupe</option>
          <option value="cabrio"        class="series-1">Cabrio</option>
          <option value="sedan"         class="series-3">Sedan</option>
          <option value="touring"       class="series-3">Touring</option>
          <option value="gran-tourismo" class="series-5">Gran Tourismo</option>

          <option value="sedan"        class="a1 a3 s3 a4 s4 a6 s6 rs6">Sedan</option>
          <option value="sportback"    class="a3 s3 a5 s5">Sportback</option>
          <option value="cabriolet"    class="a3 a5 s5">Cabriolet</option>
          <option value="avant"        class="a4 s4 a6 s6 rs6">Avant</option>
          <option value="allroad"      class="a4 a6">Allroad</option>
          <option value="coupe"        class="a5 s5">Coupe</option>
        </select>
        <button id="button" type="submit">Button</button>
      </form>
    </div>
  </div>
  <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js" type="text/javascript" charset="utf-8"></script>
  <script src="jquery.chained.js?v=1.0.0" type="text/javascript" charset="utf-8"></script>
  <script src="jquery.chained.remote.js?v=1.0.0" type="text/javascript" charset="utf-8"></script>
  <script type="text/javascript" charset="utf-8">
  $(function() {

    /* For jquery.chained.js */
    $("#series").chained("#mark");
    $("#model").chained("#series");
    /* Show button after each pulldown has a value. */
    $("#model").bind("change", function(event) {
        if ("" != $("option:selected", this).val() && "" != $("option:selected", $("#model")).val()) {
            $("#button").fadeIn();
        } else {
            $("#button").hide();
        }
    });

  });
  </script>